The present invention relates to a novel process for oligomerization of fatty esters and acids of polyunsaturated oils. More precisely, it is concern with a process for the production of dimers and trimers at low temperatures and with small quantities of catalyst from any polyunsaturated ester or acid whose double bonds are conjugated or conjugatable.
Dimers or trimers obtained from acids or esters may be used as base materials in the production of lubricants, polyamide monomers, amide type anticorrosive agents, or paint additives.
The preparation of fatty acid dimers or oligomers from monoenyl, dienyl or trienyl fatty esters or acids is known. Activated earths (also called "activated clays") containing various additives (water, acid, chlorinated derivative, lithium salt, etc.) are currently employed as catalysts. British patent document GB 2 172 597 and the article published by J C COWAN in J.A.O.C.S. Vol 39, 1962, pages 534-545, describe the syntheses. The disadvantages of this technique are known. The reaction requires from 6 to 10% by weight of activated earth with respect to the product to be oligomerized and this earth is lost. Temperatures of 220.degree. to 250.degree. C. and reaction times of 6 to 8 hours are required. Filtration is difficult. When starting from polyenyl structures, the products obtained are in part aromatic. The cloud point of these compounds produced from a polyenyl acid is not very low. It is thus preferred to use monoenyl extracts of tallow. Finally, the resulting acidity when starting from esters is often high, whatever the ester employed.
Lubricant bases cannot be manufactured using this synthesis. Fatty acids form by ester hydrolysis which necessitates that the fatty acids be esterified before any hydrogenation that may be required. In addition, the monomers must be distilled off since not only do these monomers have low molecular weights but, even in the absence of hydrogenation, a large quantity of saturated derivatives is also produced during the course of the reaction. The esters of the dimers and oligomers are then transesterified with a higher alcohol if starting from methyl esters. The manufacture of a lubricant base is thus a relatively lengthy process, and it would be of advantage to simplify the process.
The literature records attempts to manufacture diners or oligomers at low temperature (120.degree.-180.degree. C.) (French patent FR 2 202 874). However, the process used does not appear to be very attractive. The ion exchange resin used, which is only active towards dienes and trienes, gives rise to a significant amount of ester hydrolysis despite the relatively low temperature (130.degree.-140.degree. C.), bringing the acidity to acid values which may reach from 12 to 30. The product obtained is quite colored and the resin rapidly loses activity. Finally, the dimer/trimer weight ratio is close to 1, while a good value generally obtained with activated earths is on the order of 4 to 5. Dimers rather than trimers are in demand for numerous applications.
In one example in the literature, low temperature oligomerization of fatty acid esters is effected using almost 10% by weight of activated earth; however, in this instance the esters of very rare oils are used, such as esters of China wood oil or oiticica oil (British patent document GB-A 1 466 418). These esters are double bonded trienes with trans-configurations. These structures are deemed to be very polymerizable.
Finally, German patent document DE 1 268 616 mentions the use of conjugated polyunsaturated esters in a co-oligomerization reaction of esters and coumarone or indene in the presence of activated earths between 40.degree. and 180.degree. C. In this patent document, it is indicated that there is no homopolymerization of the conjugated esters since the saponification values of the products which are not monomers, after distillation, are very different from those of the dimers.